Ceramic composition for microwave dielectric

ABSTRACT

Disclosed herein is a ceramic composition which gives a microwave dielectric having a permittivity and unloaded Q in a practical range and a small temperature coefficient (τf) in the neighborhood of zero of resonance frequency owing to the controlled amount of ZnO and Ta 2  O 5  added. The τf value may be positive or negative in the neighborhood of zero according to need. The ceramic composition comprises a principal component represented by (BaO-3.7TiO 2 ).xZnO, where x is in the range of 15≦x≦17 wt % and a secondary component containing 2-6 wt % Ta 2  O 5  and not more than 1 wt % MnO 2  based on the amount of the principal component. The ceramic composition may also contain BaO-3.8TiO 2 , 12-16 wt % ZnO, and 2-6 wt % Ta 2  O 5 , or contain BaO-3.6TiO 2 , 17-19 wt % ZnO, and 2-6 wt % Ta 2  O 5 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ceramic composition for a microwave dielectric (simply referred to as a dielectric ceramic composition), said microwave dielectric having a temperature coefficient of resonance frequency (simply referred to as τf hereinafter) which is close to zero or has any controlled positive or negative value depending on the amount of ZnO added.

The microwave dielectric pertaining to the present invention will be used for microwave dielectric resonators, microwave IC substrates, and impedance matching in microwave circuits etc.

2. Description of the Prior Art

There are several known dielectric ceramic compositions which are represented by BaO.3.9-4.1TiO₂ . xZnO (Japanese Patent Publication No. 37807/1989), BaO.4TiO₂.x ZnO.yTa₂ O₅ (Japanese Patent Laid-open No. 10806/1986), BaO.4TiO₂.x ZnO.yWO3 (Japanese Patent Laid-Open No. 10807/1986), and BaO.wTiO₂.xBa(Zr_(1/3) Ta_(2/3))O₃.yTa₂ O₅.zMnO₂ (Japanese Patent Laid-open No. 117957/1988).

Of the above-mentioned four dielectric ceramic compositions, the first three vary in τf (which ranges from positive to negative) depending on the amount of additives. However, they are often poor in sinterability, and there has been a demand for a new dielectric ceramic composition of different make-up which is comparable or superior to the conventional ones in performance. And the last one does not have negative τf values.

SUMMARY OF THE INVENTION

The present invention was completed in view of the foregoing. It is an object of the present invention to provide a dielectric ceramic composition for a microwave dielectric which has a small τf value close to zero, or any desired positive or negative τf value depending on the amount of ZnO and Ta₂ O₅ added, while maintaining the permittivity (simply referred to as εr hereinafter) and unloaded Q (simply referred to as Qu hereinafter) in a practical range.

The present inventors carried out a series of researches on a variety of dielectric ceramic compositions of (BaO-xTiO₂).yZnO.zTa₂ O₅ systems which has a small τf value close to zero, while maintaining εr and Qu in a practical range. As the result, the present invention was accomplished.

In a first preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO₂ and ZnO, and a secondary component, said principal component being represented by (BaO-3.8TiO₂).xZnO, where x is in the range of 12≦x≦16 wt % based on the amount of BaO-3.8TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.

In a second preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO₂ and ZnO, and a secondary component, said principal component being represented by (BaO-3.7TiO₂).xZnO, where x is in the range of 15≦x≦17 wt % based on the amount of BaO-3.7TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.

In a third preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO₂ and ZnO, and a secondary component, said principal component being represented by (BaO-3.6TiO₂).xZnO, where x is in the range of 17≦x≦19 wt % based on the amount of BaO-3.6TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.

In a fourth preferred embodiment of the present invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO₂ and ZnO, and a secondary component, said principal component being represented by (BaO-3.7˜3.8TiO₂).xZnO, where x is in the range of 12≦x≦17 wt % based on the amount of BaO-3.7˜3.8TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component, and τf values is in the range of -5 to +5 ppm/°C.

In a fifth preferred embodiment of the invention, a ceramic composition for microwave dielectric comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.6˜3.7TiO₂).xZnO, where x is in the range of 15≦x≦19 wt % based on the amount of BaO-3.6˜3.7TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component, and a temperature coefficient of resonance frequency τf values is in the range of -5 to +5 ppm/°C.

In a sixth preferred embodiment of the invention, a ceramic composition for microwave dielectric as defined in any of the first to fifth embodiments, further comprises MnO₂ in an amount not more than 1 wt % of the amount of the ceramic composition.

The dielectric ceramic composition of the present invention is represented by [(BaO.xTiO₂).yZnO].zTa₂ O₅, with x, y, and z having specific values as defined in the first to fifth embodiments, so that the resulting microwave dielectric has the τf value close to zero, while maintaining the Qu and εr values in a practical range (Qu≧2200, εr≧26). The τf value ranges from +4.9 to -1.7 in the first embodiment, from +2.5 to -1.4 in the second embodiment, from +4.1 to -1.0 in the third embodiment and from +5 to -5 in the fourth and fifth embodiments.

The composition contains ZnO which improves Qu and εr and also contains Ta₂ O₅ which improves Qu and permits the control of τf.

The composition may optionally contain MnO₂ which permits the composition to be sintered stably at a comparatively low temperature. The amount of MnO₂ in not more than 1 wt % has effect enough. The amount of MnO₂ in excess of 1 wt % has an adverse effect on the dielectric properties.

According to the present invention, the dielectric ceramic composition is prepared by mixing BaO, TiO₂, ZnO, Ta₂ O₅, and MnO₂ (all in powder form) according to the formula and calcining the mixture. The calcined powder is formed into a desired shape, which is subsequently sintered.

According to the present invention, the dielectric ceramic composition is represented by any one of the following formulas:

1[BaO-3.8 (mol) TiO₂.x (wt %) ZnO (12≦x≦16)].2-6 (wt %) Ta₂ O₅.0.2 (wt %) MnO₂, or 2[BaO-3.7 (mol) TiO₂.x (wt %) ZnO (15≦x≦17)].2-6 (wt %) Ta₂ O₅.0.2 (wt %) MnO₂, or 3[BaO-3.6 (mol) TiO₂.x (wt %) ZnO (17≦x≦19)].2-6 (wt %) Ta₂ O₅.0.2 (wt %) MnO₂,

It gives rise to a microwave dielectric which has a τf value close to zero while maintaining εr and Qu in a practical range. It also has a desired positive or negative τf value depending on the amount of ZnO or Ta₂ O₅ added.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.8TiO₂ -based dielectric ceramic composition.

FIG. 2 is a graph showing the relationship between the τf value and the amount of Ta₂ O₅ at different sintering temperatures for the BaO-3.8TiO₂ -based dielectric ceramic composition.

FIG. 3 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.7TiO₂ -based dielectric ceramic composition.

FIG. 4 is a graph showing the relationship between the τf value and the amount of Ta₂ O₅ at different sintering temperatures for the BaO-3.7TiO₂ -based dielectric ceramic composition.

FIG. 5 is a graph showing the relationship between the τf value and the amount of ZnO at different sintering temperatures for the BaO-3.6TiO₂ -based dielectric ceramic composition.

FIG. 6 is a graph showing the relationship between the τf value and the amount of Ta₂ O₅ at different sintering temperatures for the BaO-3.6TiO₂ -based dielectric ceramic composition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in more detail with reference to the following example.

(1) Preparation of samples

Five starting materials, BaCO₃, TiO₂, ZnO, Ta₂ O₅, and MnO₂, all in powder form and 99.9% pure, were weighed according to the formula below.

[BaO-y (mol) TiO₂.x (wt %) ZnO].z (wt %) Ta₂ O₅.0.2 (wt %) MnO₂ where:

when y is 3.8, x is 12, 14, or 16, and z is 4, as shown in Table 1;

when y is 3.8 and x is 14, z is 2, 4, or 6, as shown in Table 2;

when y is 3.7, x is 15, 16, or 17, and z is 4, as shown in Table 3;

when y is 3.7 and x is 17, z is 2, 4, or 6, as shown in Table 4;

when y is 3.6, x is 17, 18, or 19, and z is 4, as shown in Table 5; and

when y is 3.6 and x is 19, z is 2, 4, or 6, as shown in Table 6.

Incidentally, each amount of ZnO, Ta₂ O₅, and MnO₂ is expressed in terms of wt % (parts by weight) of the total amount (100 parts by weight) of BaCO₃ -yTiO₂. They were mixed and crushed by dry process using a mixer, and the mixture was calcined at 900°-1100° C. for 2 hours. The calcined product was crushed together with an adequate amount of organic binder and 260-500 g of deionized water in a ball mill containing alumina balls (20 mm in diameter). The crushed product was granulated by spray drying. The granules were formed into a cylinder, 19.35 mm in diameter and 8 mm high, by pressing at 1000 kg/cm².

                                      TABLE 1                                      __________________________________________________________________________     (BaO-3.8TiO.sub.2 -based dielectric ceramic composition)                                    Firing                                                                              Firing                                                                              Relative                                                                             Qu value                                                                            Temperature                                  Sample                                                                             Additives (wt %)                                                                        tempera-                                                                            density                                                                             permittivity                                                                         (at  coefficient                                  No. ZnO Ta.sub.2 O.sub.5                                                                    ture (° C.)                                                                  (g/cm.sup.3)                                                                        (ε.sub.r)                                                                    4.5 GHz)                                                                            (τf) ppm/°C.                      __________________________________________________________________________     1   12  4    1175 3.81 23.1   700 7.80                                         2   14  4    1175 4.54 30.9  3270 -0.57                                        3   16  4    1175 3.94 23.8  1350 -1.69                                        4   12  4    1200 4.45 30.4  3040 3.85                                         5   14  4    1200 4.68 32.3  3250 0.00                                         6   16  4    1200 4.51 30.1  3200 -0.74                                        7   12  4    1225 4.54 31.3  3120 2.89                                         8   14  4    1225 4.77 33.1  3050 0.70                                         9   16  4    1225 4.58 31.2  2860 -0.50                                        10  12  4    1250 4.71 33.0  2840 2.57                                         11  14  4    1250 4.56 33.4  2840 0.21                                         12  16  4    1250 4.74 32.4  2670 -0.46                                        __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     (BaO-3.8TiO.sub.2 -based dielectric ceramic composition)                           Additives                                                                             Firing                                                                              Firing                                                                              Relative        Temperature                               Sample                                                                             (wt %) tempera-                                                                            density                                                                             permittivity                                                                           Qu value                                                                               coefficient                               No. ZnO                                                                               Ta.sub.2 O.sub.5                                                                   ture (° C.)                                                                  (g/cm.sup.3)                                                                        (ε.sub.r)                                                                      (at 4.5 GHz)                                                                           (τf) ppm/°C.                   __________________________________________________________________________     13  14 2   1175 3.99 25.1     530    7.30                                      14  14 4   1175 4.54 30.9    3270    -0.57                                     15  14 6   1175 3.89 Unmeasurable                                                                           Unmeasurable                                                                           Unmeasurable                              16  14 2   1200 4.55 30.8    3190    1.34                                      17  14 4   1200 4.68 32.3    3250    0.00                                      18  14 6   1200 4.43 29.6    3140    -0.09                                     19  14 2   1225 4.64 31.7    2700    1.13                                      20  14 4   1225 4.77 33.1    3050    0.70                                      21  14 6   1225 4.60 31.1    3020    -1.09                                     22  14 2   1250 4.72 32.6    2330    1.37                                      23  14 4   1250 4.56 33.4    2840    0.21                                      24  14 6   1250 4.73 32.6    2550    -0.09                                     __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     (BaO-3.7TiO.sub.2 -based dielectric ceramic composition)                           Additives                                                                             Firing                                                                              Firing                                                                              Relative        Temperature                               Sample                                                                             (wt %) tempera-                                                                            density                                                                             permittivity                                                                           Qu value                                                                               coefficient                               No. ZnO                                                                               Ta.sub.2 O.sub.5                                                                   ture (° C.)                                                                  (g/cm.sup.3)                                                                        (ε.sub.r)                                                                      (at 4.5 GHz)                                                                           (τf) ppm/°C.                   __________________________________________________________________________     25  15 4   1250 4.85 33.7    2800    2.34                                      26  16 4   1250 4.81 33.0    2550    0.77                                      27  17 4   1250 4.84 32.8    2600    -0.27                                     28  15 4   1225 4.82 33.6    2780    2.44                                      29  16 4   1225 4.73 32.2    2600    1.31                                      30  17 4   1225 4.83 32.7    2750    -0.16                                     31  15 4   1275 4.83 33.7    2680    1.35                                      32  16 4   1275 4.82 33.2    2760    1.18                                      33  17 4   1275 4.86 33.0    2510    -1.34                                     34  15 4   1200 4.68 32.2    3040    1.69                                      35  16 4   1200 4.30 27.7    2950    0.00                                      36  17 4   1200 4.77 32.0    2960    -0.80                                     37  15 4   1175 4.54 30.6    2780    1.46                                      38  16 4   1175 4.11 Unmeasurable                                                                           Unmeasurable                                                                           Unmeasurable                              39  17 4   1175 4.62 30.6    3210     -1.16                                    __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________     (BaO-3.7TiO.sub.2 -based dielectric ceramic composition)                           Additives                                                                              Firing                                                                              Firing                                                                              Relative                                                                             Qu value                                                                            Temperature                                   Sample                                                                             (wt %)  tempera-                                                                            density                                                                             permittivity                                                                         (at  coefficient                                   No. ZnO Ta.sub.2 O.sub.5                                                                   ture(° C.)                                                                   (g/cm.sup.3)                                                                        (ε.sub.r)                                                                    4.5 GHz)                                                                            (τf) ppm/°C.                       __________________________________________________________________________     40  17  2   1175 4.13 25.9  2210 0.25                                          41  17  4   1175 4.62 30.6  3210 -1.16                                         42  17  6   1175 4.00 22.7  1420 -7.84                                         43  17  2   1200 4.30 30.5  2690 1.02                                          44  17  4   1200 4.77 32.0  2960 -0.80                                         45  17  6   1200 4.44 28.3  2720 -2.37                                         46  17  2   1225 4.62 31.3  2830 0.99                                          47  17  4   1225 4.83 32.7  2750 -0.16                                         48  17  6   1225 4.64 30.4  3230 -1.51                                         49  17  2   1250 4.75 32.7  2540 3.36                                          50  17  4   1250 4.84 32.8  2600 -0.27                                         51  17  6   1250 4.84 32.4  2560 -1.60                                         __________________________________________________________________________

                                      TABLE 5                                      __________________________________________________________________________     (BaO-3.6TiO.sub.2 -based dielectric ceramic composition)                           Additives                                                                             Firing                                                                              Firing                                                                              Relative        Temperature                               Sample                                                                             (wt %) tempera-                                                                            density                                                                             permittivity                                                                           Qu value                                                                               coefficient                               No. ZnO                                                                               Ta.sub.2 O.sub.5                                                                   ture (°C.)                                                                   (g/cm.sup.3)                                                                        (ε.sub.r)                                                                      (at 4.5 GHz)                                                                           (τf) ppm/°C.                   __________________________________________________________________________     52  17 4   1250 4.84 33.0    2600    3.54                                      53  18 4   1250 4.84 32.6    2590    1.71                                      54  19 4   1250 4.85 32.2    2490    -0.61                                     55  17 4   1225 4.77 32.4    2780    4.07                                      56  18 4   1225 4.79 32.1    2300    0.55                                      57  19 4   1225 4.79 31.6    2530    -0.98                                     58  17 4   1275 4.85 33.2    2460    3.39                                      59  18 4   1275 4.85 32.9    2690    2.12                                      60  19 4   1275 4.86 32.4    2560    -0.39                                     61  17 4   1200 4.59 30.5    2990    2.67                                      62  18 4   1200 4.65 30.8    2920    0.95                                      63  19 4   1200 4.54 29.3    2900    -0.35                                     64  17 4   1175 4.32 Unmeasurable                                                                           Unmeasurable                                                                           Unmeasurable                              65  18 4   1175 4.50 29.2    1700    2.44                                      66  19 4   1175 4.30 Unmeasurable                                                                           Unmeasurable                                                                            Unmeasurable                             __________________________________________________________________________

                                      TABLE 6                                      __________________________________________________________________________     (BaO-3.6TiO.sub.2 -based dielectric ceramic composition)                           Additives                                                                             Firing                                                                              Firing                                                                              Relative        Temperature                               Sample                                                                             (wt %) tempera-                                                                            density                                                                             permittivity                                                                           Qu value                                                                               coefficient                               No. ZnO                                                                               Ta.sub.2 O.sub.5                                                                   ture (° C.)                                                                  (g/cm.sup.3)                                                                        (ε.sub.r)                                                                      (at 4.5 GHz)                                                                           (τf) ppm/°C.                   __________________________________________________________________________     67  19 2   1175 4.36 27.8    1540    0.78                                      68  19 4   1175 4.30 Unmeasurable                                                                           Unmeasurable                                                                           Unmeasurable                              69  19 6   1175 4.10 23.7    1740    -2.41                                     70  19 2   1200 4.68 31.0    2650    0.72                                      71  19 4   1200 4.54 29.3    2900    -1.35                                     72  19 6   1200 4.61 29.4    3090    -1.49                                     73  19 2   1225 4.72 31.4    2810    0.59                                      74  19 4   1225 4.79 31.6    2530    -0.98                                     75  19 6   1225 4.73 30.7    2940    -0.72                                     76  19 2   1250 4.78 32.1    2480    0.46                                      77  19 4   1250 4.85 32.2    2490    -0.61                                     78  19 6   1250 4.86 32.0    2900    -0.78                                     __________________________________________________________________________

The molded article was fired in the air at 1175°-1275° C. for 4 hours. Finally, the fired article was polished to give a cylindrical article, 16 mm in diameter and 6 mm in height. Thus there were obtained dielectric samples Nos. 1 to 78.

The samples were tested for εr, Qu and τf by the parallel conductor plate type dielectric resonator method (TE₀₁₁ MODE) at a resonant frequency of 4.5 GHz. They were also tested for sintering density.

(2) Evaluation of Performance

The characteristic properties of the samples are shown in Tables 1 to 6 and FIGS. 1 to 6. Incidentally, Tables 1 and 2 and FIGS. 1 and 2 are concerned with BaO-3.8TiO₂ -based Tables 3 and 4 and FIGS. 3 and 4 are concerned with BaO-3.7TiO₂ -based compositions; and Tables 5 and 6 and FIGS. 5 and 6 are concerned with BaO-3.6TiO₂ compositions.

These results indicate the following.

The BaO-3.8TiO₂ -based samples (in Tables 1 and 2 and FIGS. 1 and 2) gave τf values in the range of -1.09 to +3.85 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta₂ O₅ increases. It is a positive value close to zero when the amount of Ta₂ O₅ is 4 wt % and the amount of ZnO is 14 wt %. In other words, it is close to zero when the amount of ZnO is 14-16 wt % and the amount of Ta₂ O₅ is 4-6 wt %.

The BaO-3.7TiO₂ -based samples (in Tables 3 and 4 and FIGS. 3 and 4) gave τf values in the range of -2.37 to +3.36 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta₂ O₅ increases. It is a negative value close to zero when the amount of Ta₂ O₅ is 4 wt % and the amount of ZnO is 17 wt %. In other words, it is close to zero when the amount of ZnO is 16-17 wt % and the amount of Ta₂ O₅ is 2-4 wt %.

The BaO-3.6TiO₂ -based samples (in Tables 5 and 6 and FIGS. 5 and 6) gave τf values in the range of -1.49 to +3.54 ppm/°C., which are small and close to zero. The τf value changes from positive to negative as the amount of ZnO and Ta₂ O₅ increases. It is a negative value close to zero when the amount of Ta₂ O₅ is 4 wt % and the amount of ZnO is 19 wt %. In other words, it is close to zero when the amount of ZnO is 18-19 wt % and the amount of Ta₂ O₅ is 2-4 wt %.

Moreover, it is noted that Qu tends to decrease and εr tends to increase with the increasing firing temperature. The BaO-3.8TiO₂ -based samples gave Qu values in the measurable range of 2330-3250, εr values in the range of 29.55-33.41, and firing densities in the range of 4.43-4.77 g/cm³ (high enough for practical use).

The BaO-3.7TiO₂ -based samples gave Qu values in the measurable range of 2510-3210, εr values in the range of 27.7-33.7, and firing densities in the range of 4.30-4.86 g/cm³ (except sample No. 38, high enough for practical use). In addition, all the samples (except No. 38) gave sufficiently compact sintered bodies even at comparatively low firing temperatures, 1175° C. and 1200° C. Incidentally, εr values tend to be low when the firing density is low. Presumably, this is due to incomplete densification.

The BaO-3.6TiO₂ -based samples gave Qu values in the measurable range of 2300-2990 (except sample No. 65 fired at 1175° C.), εr values in the measurable range of 29.2-33.2, and firing densities in the measurable range of 4.50-4.86 g/cm³ (except samples Nos. 64 and 66, high enough for practical use). In addition, all the samples (except Nos. 64 and 66) gave sufficiently compact sintered bodies even at comparatively low firing temperatures, 1175° C. and 1200° C.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. In other words, the calcination and firing may be carried out under various conditions, and the BaCO₃ as a raw material of BaO may be replaced by a peroxide, hydroxide, or nitrate. 

What is claimed is:
 1. A ceramic composition for microwave dielectric which comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.8TiO₂).xZnO, where x is in the range of 12≦x≦16 wt % based on the amount of BaO-3.8TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.
 2. A ceramic composition for microwave dielectric which comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.7TiO₂).xZnO, where x is in the range of 15≦x≦17 wt % based on the amount of BaO-3.7TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.
 3. A ceramic composition for microwave dielectric which comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.6TiO₂).xZnO, where x is in the range of 17≦x≦19 wt % based on the amount of BaO-3.6TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component.
 4. A ceramic composition for microwave dielectric which comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.7˜3.8TiO₂).xZnO, where x is in the range of 12≦x≦17 wt % based on the amount of BaO-3.7˜3.8TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component, and a temperature coefficient of resonance frequency τf values is in the range of -5 to +5 ppm/°C.
 5. A ceramic composition for microwave dielectric which comprises a principal component composed of BaO, TiO₂, and ZnO, and a secondary component, said principal component being represented by (BaO-3.6˜3.7TiO₂).xZnO, where x is in the range of 15≦x≦19 wt % based on the amount of BaO-3.6˜3.7TiO₂, said secondary component containing 2-6 wt % Ta₂ O₅ based on the amount of the principal component, and a temperature coefficient of resonance frequency τf values is in the range of -5 to +5 ppm/°C.
 6. A ceramic composition for microwave dielectric as defined in any of claims 1 to 5, which further comprises MnO₂ in an amount not more than 1 wt % of the amount of the ceramic composition. 